Aquaculture can bring excess phosphorus to lakes which causing eutrophication. Figuring out the amount of phosphorus is meaningful to water pollution control. This study aims to clarify how much phosphorus is produced by aquaculture in Baiyangdian Lake each year. A phosphorus balance equation which is based on conservation of matter is established and used to calculate the input and output phosphorus carefully of an experimental field during a breeding cycle. Six parameters which are little fish, adult fish, feed, plants, sediment deposition and release are included in the equation. By the equation, the document gets a model: Q(x)＝0.033x, which shows that breeding every 1kg grass carp, there will be 0.033kg phosphorus into the water. As a result, the phosphorous pollution load of aquaculture in Baiyangdian Lake is 302.6 t/a.